Time#in_milliseconds preserves millisecond precision

`Time` stores the number of nanoseconds since the Epoch.
`Time#in_milliseconds` should return the number of milliseconds since the
Epoch with a millisecond accuracy.
```ruby
require 'benchmark'

INT = 12345678
FLOAT = 1234.5678

INT_ARG = 1000
FLOAT_ARG = 1000.0

TIMES = 10_000_000

Benchmark.bmbm do |x|
  x.report("int_to_f_and_float_argument") { TIMES.times { INT.to_f * FLOAT_ARG } }
  x.report("int_to_f") { TIMES.times { INT.to_f * INT_ARG } }
  x.report("int_float_argument") { TIMES.times { INT * FLOAT_ARG } }

  x.report("float_to_f_and_float_argument") { TIMES.times { FLOAT.to_f * FLOAT_ARG } }
  x.report("float_to_f") { TIMES.times { FLOAT.to_f * INT_ARG } }
  x.report("float_float_argument") { TIMES.times { FLOAT * FLOAT_ARG } }  
end
```

```
Rehearsal -----------------------------------------------------------------
int_to_f_and_float_argument     2.640000   0.010000   2.650000 (  2.639056)
int_to_f                        3.130000   0.000000   3.130000 (  3.130642)
int_float_argument              1.720000   0.000000   1.720000 (  1.727530)
float_to_f_and_float_argument   1.780000   0.000000   1.780000 (  1.777205)
float_to_f                      2.330000   0.000000   2.330000 (  2.329920)
float_float_argument            1.480000   0.000000   1.480000 (  1.486658)
------------------------------------------------------- total: 13.090000sec

                                    user     system      total        real
int_to_f_and_float_argument     2.640000   0.000000   2.640000 (  2.646779)
int_to_f                        3.500000   0.010000   3.510000 (  3.503105)
int_float_argument              1.950000   0.000000   1.950000 (  1.948127)
float_to_f_and_float_argument   1.910000   0.000000   1.910000 (  1.912072)
float_to_f                      2.450000   0.000000   2.450000 (  2.454444)
float_float_argument            1.770000   0.000000   1.770000 (  1.776772)
```

Whether the receiver is integer or float, the quickest implementation is to *not* `to_f` it, but instead to multiply it by a float argument.